For jobs still queued, we ask the queue to change the priority,
and replicate the changed job. For jobs that have already been
started, we have to contact the job directly, which, at the moment,
means forwarding the request to masterd.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>